Prepare the marinade: In a gallon-sized ziptop bag, whisk together garlic, adobo sauce, olive oil, lime juice, chili powder, cumin, oregano, smoked paprika, and salt. This mixture will infuse the steak with flavor, so ensure all ingredients are well incorporated.
Marinate the steak: Place the steak into the bag, seal it tightly, and refrigerate for at least 30 minutes—or even up to 24 hours if you have the time. The longer it marinates, the more flavorful it will be.
Prepare vegetables: Heat 1 tablespoon of olive oil in a large cast iron skillet over medium-high heat. Add the sliced green bell pepper and red onion. Sauté until they’re lightly charred but still maintain a bit of crunch, about 3 to 4 minutes. Season with salt to taste. Once done, remove the vegetables from the skillet and set aside.
Cook the steak: Take the steak out of the fridge about 30 minutes before cooking to ensure it comes to room temperature. Add a tablespoon of olive oil to the skillet over medium-high heat. Sear the steak for about 3 minutes per side, or until a nice crust forms, and the internal temperature reaches 125 to 135 degrees Fahrenheit for medium-rare to medium. Transfer the steak to a cutting board and let it rest for 5 minutes before cutting it into small cubes.
Assemble quesadillas: Wipe the skillet again and set it over medium heat. Lay one tortilla flat in the skillet, add a third of the sautéed vegetables on one side, then a third of the steak cubes, and sprinkle with ¼ cup of cheese. Fold the tortilla in half over the filling.
Cook quesadillas: Cook until the tortilla is crisp and the cheese has melted, about 2 minutes per side. Repeat with the remaining tortillas and ingredients. Once cooked, remove the quesadillas, cut each into two pieces, and serve warm with a dollop of sour cream.